Hydrodynamic bearing and disc rotation apparatus using the same

ABSTRACT

A high-accuracy, long-life hydrodynamic bearing that does not cause oil film breakage in bearing clearances and a disc rotation apparatus using the bearing is disclosed. Oil film breakage is avoided as negative pressure is prevented from generating between the shaft and sleeve of the hydrodynamic bearing. Herringbone shaped dynamic pressure generating grooves, located on the thrust bearing section and the radial bearing section of the hydrodynamic bearing, are oil filled and have optimum shapes. The optimum shapes prevent the generation negative pressure and thus prevents the coagulation of air bubbles that can cause oil film breakage. The disc rotation apparatus, that holds a reproduction/recording disc, is concentrically secured to the hydrodynamic bearing and rotated. The disc is put into contact with magnetic or optical heads while rotating in the disc rotation apparatus. Both the hydrodynamic bearing and the disc rotation apparatus experience high reliability.

BACKGROUND ART

1. Technical Field

The present invention relates to a hydrodynamic bearing having fluid inits rotation section and a disc rotation apparatus having the same.

2. Prior Art

In recent years, in recording apparatuses using discs and the like,their memory capacities are increasing and their data transfer speedsare rising. Hence, a disc rotation apparatus for use in this kind ofrecording apparatus is required to rotate at high speed and with highaccuracy, and a hydrodynamic bearing is used in its rotating main shaftsection.

A conventional hydrodynamic bearing and an example of a disc rotationapparatus having the same will be described below referring to FIG. 12.FIG. 12 is a cross-sectional view showing the right portion of thecenter of the rotation shaft, that is, the center line C, of theconventional hydrodynamic bearing. In FIG. 12, a shaft 31 is rotatablyinserted into a sleeve 32 having a bearing hole 32A. At the lower end ofthe shaft 31, a flange 33 is provided so as to be integrated therewith.The lower end of the flange 33 is accommodated in a recess portionformed by a hole in a base 35 and the sleeve 32 and rotatably held so asto be opposed to a thrust plate 34 mounted on the base 35. A hub rotor36, a rotor magnet 38, a plurality of discs 39, spacers 40 and a clamper41 are secured to the shaft 31. A motor stator 37 opposed to the rotormagnet 38 is installed on the base 35. Dynamic pressure generationgrooves 32B and 32C indicated by broken lines are provided on the innercircumferential face of the bearing hole 32A of the sleeve 32. Dynamicpressure generation grooves 33A are provided on the upper face of theflange 33, a face opposed to the sleeve 32. In addition, dynamicpressure generation grooves 33B are provided on the lower face of theflange 33, a face opposed to the thrust plate 34. The clearances betweenthe shaft 31 and the sleeve 32, including the dynamic pressuregeneration grooves 32B, 32C, 33A and 33B, are filled with oil.

The operation of the conventional hydrodynamic bearing shown in FIG. 12will be described below. In FIG. 12, when electric power is applied tothe coil of the stator 37, a rotating magnet field is generated, and arotation force is generated in the rotor magnet 38, whereby the shaft 31and the flange 33 rotate together with the hub rotor 36 and the discs39. During the rotation, dynamic pressures are generated in the oil bythe dynamic pressure generation grooves 32B, 32C, 33A and 33B, and theshaft 31 is floated in the upward direction of the figure and rotateswhile holding space from the sleeve 32 and without making contact withthe thrust plate 34 and the sleeve 32. Magnet heads, not shown, makecontact with the discs 39 and carry out the recording and reproductionof electrical signals.

The conventional hydrodynamic bearing configured as described above hadproblems described below. FIG. 13 is a plan view of the flange 33 whichis provided with a plurality of the dynamic pressure generation grooves33A indicated by black-colored regions. FIG. 14 is a bottom view of theflange 33 which is similarly provided with a plurality of the dynamicpressure generation grooves 33B indicated by black-colored regions. Theoutside diameters of the patterns of the dynamic pressure generationgrooves 33A and 33B on the top and bottom faces are represented by D1 oand D2 o, respectively, and their inside diameters are represented by D1i and D2 i, respectively. The diameters D1 m and D2 m of the respectiveturn-back parts of the dynamic pressure generation grooves 33A and 33Bare set at sufficiently large values so that pumping pressures in thedirections indicated by arrows E and F and by arrows G and H,respectively, are raised.

FIG. 15 and FIG. 16 are views showing the cross sections of relevantparts in the vicinity of the lower end of the shaft 31 and showingpressures on the surfaces of the flange 33 and the shaft 31 of theabove-mentioned conventional hydrodynamic bearing. If the pumpingpressures in the directions indicated by arrows E and H shown in FIG. 13and FIG. 14, respectively, are raised too high, a negative pressure withrespect to atmospheric pressure is generated at the central portion ofthe lower face of the flange 33 as indicated by curve P1 in FIG. 15,whereby air bubbles mixed in the oil are coagulated and air isaccumulated in a region 43B having a constant size.

In FIG. 16, the dynamic pressure generation grooves 32B and 32C of thesleeve 32 are made so that dimension L1 in the figure is larger thandimension L2, (L1>L2), and so that dimension L4 is larger than dimensionL3, (L4>L3). In addition, the dimensional difference (L1−L2) is selectedso as to be nearly equal to the dimensional difference (L4−L3), that is,(L1−L2)≈(L4−L3). As shown by ΔL in FIG. 16, in the case that the amountof the oil becomes slightly insufficient and the upper face of the oilis at the position lower than the upper ends of the dynamic pressuregeneration grooves 33B by dimension 4L, no oil is present in the portioncorresponding to the dimension ΔL of the upper ends of the dynamicpressure generation grooves 33B, whereby the pressure distribution ofoil is represented by curve P2 shown in FIG. 16. In addition, a negativepressure is generated at the lower portion of the range of the dimensionL4 in the figure. Hence, air bubbles are accumulated in a region 43A,whereby there is a fear of breaking the oil film in this region 43A andof causing friction between the shaft 31 and the sleeve 32.

SUMMARY OF THE INVENTION

The present invention purports to provide a hydrodynamic bearing inwhich a negative pressure is prevented from generating between the shaftand the sleeve, whereby oil film breakage due to locally accumulated airin oil does not occur.

A hydrodynamic bearing in accordance with the present inventioncomprises a sleeve having a bearing hole at the nearly central portionthereof, a shaft rotatably inserted into the bearing hole of theabove-mentioned sleeve, and a nearly disc-shaped flange, secured to oneend of the above-mentioned shaft, one face of which is opposed to theend face of the sleeve 1 and the other face of which is opposed to athrust plate provided to hermetically seal a region including theabove-mentioned end face of the above-mentioned sleeve, whereinherringbone-shaped first and second dynamic pressure generation groovesare provided on at least one of the inner circumferential face of theabove-mentioned sleeve and the outer circumferential face of theabove-mentioned shaft so as to be arranged in the direction along theshaft, herringbone-shaped third dynamic pressure generation grooves areprovided on at least one of the opposed faces of the flange and thethrust plate, the above-mentioned first, second and third dynamicpressure generation grooves are filled with oil having a kinematicviscosity of 4 cSt (centi-stokes) or more at 40° C. of temperature, oneof the above-mentioned sleeve and the above-mentioned shaft is securedto a base and the other is secured to a rotatable hub rotor, and whenthe outside diameter of the herringbone pattern of the above-mentionedthird dynamic pressure generation groove is designated as d1 o, theinside diameter thereof is designated as d1 i, the diameter of theturn-back part thereof is designated as d1 m, and the diameter of theturn-back part of the herring pattern, wherein the oil pressuregenerated by the above-mentioned third dynamic pressure generationgrooves in the direction from the outer circumference to the innercircumference of the flange becomes equal to the oil pressure generatedin the direction from the inner circumference to the outer circumferencethereof, is designated as dsy, the diameter d1 m of the turn-back partis determined so that the following two equations are satisfiedrespectively:d1m=dsy−(dsy−d1i)×Adsy={(d1i ² +d1o ²)/2}^(1/2),where A is a value in the range of 0.05 or more to less than 1.0.

A hydrodynamic bearing in accordance with another aspect of the presentinvention comprises a sleeve having a bearing hole at the nearly centralportion thereof, a shaft-rotatably inserted into the bearing hole of theabove-mentioned sleeve, and a nearly disc-shaped flange, secured to oneend of the above-mentioned shaft, one face of which is opposed to theend face of the sleeve 1 and the other face of which is opposed to athrust plate provided to hermetically seal a region including theabove-mentioned end face of the above-mentioned sleeve, whereinherringbone-shaped first and second dynamic pressure generation groovesare provided on at least one of the inner circumferential face of theabove-mentioned sleeve and the outer circumferential face of theabove-mentioned shaft so as to be arranged in the direction along theshaft, among the above-mentioned first and second dynamic pressuregeneration grooves, when the grooves away from the above-mentionedthrust plate are designated as the first dynamic pressure generationgrooves and the grooves close thereto are designated as the seconddynamic pressure generation grooves, the first length L1 of the grooveportion, away from the above-mentioned thrust plate, of theabove-mentioned herringbone-shaped first dynamic pressure generationgroove in the direction of the shaft is larger than the second length L2of the groove portion close to the above-mentioned thrust plate in thedirection of the shaft, the above-mentioned herringbone-shaped seconddynamic pressure generation groove is made symmetric with respect to aline passing through the herringbone-shaped turn-back parts, the valueof a calculation expression, (L1+L2)/(2×L2) represented by using theabove-mentioned first length L1 and the above-mentioned second lengthL2, is in the range of 1.02 to 1.60, herringbone-shaped third dynamicpressure generation grooves are provided on at least one of the opposedfaces of the flange and the thrust plate, the above-mentioned first,second and third dynamic pressure generation grooves are filled with oilhaving a kinematic viscosity of 4 cSt or more at 40° C. of temperature,and one of the above-mentioned sleeve and the above-mentioned shaft issecured to a base and the other is secured to a rotatable hub rotor.

A hydrodynamic bearing in accordance with another aspect of the presentinvention comprises a sleeve having a bearing hole at the nearly centralportion thereof, a shaft rotatably inserted into the bearing hole of theabove-mentioned sleeve, and a nearly disc-shaped flange, secured to oneend of the above-mentioned shaft, one face of which is opposed to theend face of the sleeve 1 and the other face of which is opposed to athrust plate provided to hermetically seal a region including theabove-mentioned end face of the above-mentioned sleeve, whereinherringbone-shaped first and second dynamic pressure generation groovesare provided on at least one of the inner circumferential face of theabove-mentioned sleeve and the outer circumferential face of theabove-mentioned shaft, among the above-mentioned first and seconddynamic pressure generation grooves, when the grooves away from theabove-mentioned thrust plate are designated as the first dynamicpressure generation grooves and the grooves close thereto are designatedas the second dynamic pressure generation grooves, the first length L1of the groove portion, away from the above-mentioned thrust plate, ofthe above-mentioned herringbone-shaped first dynamic pressure generationgroove in the direction of the shaft is larger than the second length L2of the groove portion close to the above-mentioned thrust plate in thedirection of the shaft, the above-mentioned herringbone-shaped seconddynamic pressure generation groove is made symmetric with respect to aline passing through the herringbone-shaped turn-back parts, the valueof a calculation expression, (L1+L2)/(2×L2) represented by using theabove-mentioned first length L1 and the above-mentioned second lengthL2, is in the range of 1.02 to 1.60, herringbone-shaped third dynamicpressure generation grooves are provided on at least one of the opposedfaces of the flange and the thrust plate, the above-mentioned first,second and third dynamic pressure generation grooves are supplied withoil having a kinematic viscosity of 4 cSt or more at 40° C. oftemperature, one of the above-mentioned sleeve and the above-mentionedshaft is secured to a base and the other is secured to a rotatable hubrotor, and when the outside diameter of the herringbone pattern of theabove-mentioned third dynamic pressure generation groove is designatedas d1 o, the inside diameter thereof is designated as d1 i, the diameterof the turn-back part thereof is designated as d1 m, and the diameter ofthe turn-back part of the herring pattern, wherein the oil pressuregenerated by the above-mentioned third dynamic pressure generationgrooves in the direction from the outer circumference to the innercircumference of the flange becomes equal to the oil pressure generatedin the direction from the inner circumference to the outer circumferencethereof, is designated as dsy, the diameter d1 m of the turn-back partis determined so that the following equations are satisfiedrespectively:d1m=dsy−(dsy−d1i)×Adsy{(d1i ² +d1o ²)/2}^(1/2),where A is a value in the range of 0.05 or more to less than 1.0.

In accordance with the above-mentioned configurations of the presentinvention, the patterns of the dynamic pressure generation grooves inthe thrust bearing section and the radial bearing section have optimumshapes, whereby no negative pressure is generated inside the bearing.Hence, since air accumulation due to the coagulation of air bubbles canbe prevented, it is possible to provide a hydrodynamic bearing notcausing oil film breakage.

A disc rotation apparatus using the hydrodynamic bearing in accordancewith the present invention records or reproduces signals, wherein arecording/reproduction disc is concentrically secured to the hub rotorof the hydrodynamic bearing in accordance with claims 1 to 4 androtated, magnetic heads or optical heads are provided so as to beopposed to the faces of the above-mentioned rotating disc, and themagnetic heads or optical heads are configured so as to be movable inparallel with the faces of the above-mentioned disc. By using thehydrodynamic bearing in accordance with the present invention, it ispossible to obtain a disc rotation apparatus being high in reliabilitylike that of the bearing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a hydrodynamic bearing in accordancewith a preferred embodiment of the present invention;

FIG. 2 is a bottom view of the flange 3 of the hydrodynamic bearing inaccordance with this embodiment;

FIG. 3 is a plan view of the flange 3 of the hydrodynamic bearing inaccordance with this embodiment;

FIG. 4 is a graph showing the relationship between the pump pressure inthe dynamic pressure generation grooves 3A of the flange 3 and thedimensional distribution of the diameter dsy of the turn-back part andthe inside diameter d1 i of the dynamic pressure generation groove 3A inthe hydrodynamic bearing in accordance with this embodiment;

FIG. 5 is a cross-sectional view of a relevant part showing thedistribution of oil pressure generated by the dynamic pressuregeneration grooves 3A and 3B in the case when the floating distance S1between the flange 3 and the thrust plate 4 is sufficiently small in thehydrodynamic bearing in accordance with this embodiment;

FIG. 6 is a cross-sectional view of a relevant part showing thedistribution of oil pressure generated by the dynamic pressuregeneration grooves 3A and 3B in the case when the floating distance S2between the flange 3 and the thrust plate 4 is sufficiently large in thehydrodynamic bearing in accordance with this embodiment;

FIG. 7 is a cross-sectional view of a relevant part showing thedistribution of oil pressure in the radial bearing section and thedistribution of oil pressure generated by the dynamic pressuregeneration grooves 3A and 3B of the flange 3 of the hydrodynamic bearingin accordance with this embodiment;

FIG. 8 is a cross-sectional view of a relevant part showing thedistribution of oil pressure in the radial bearing section and thedistribution of oil pressure generated by the dynamic pressuregeneration grooves 3A and 3B of the flange 3 in the case when the amountof oil is smaller than a specified amount in the hydrodynamic bearing inaccordance with this embodiment;

FIG. 9 is a graph showing the relationship between the oil pressuregenerated by the dynamic pressure generation grooves 1A and 1B and thedimensional distribution of the dynamic pressure generation grooves 1Aand 1B of the hydrodynamic bearing in accordance with this embodiment;

FIG. 10 is a graph showing the relationship between the bubble entryamount in oil and the kinematic viscosity of oil at 40° C. oftemperature in the hydrodynamic bearing in accordance with thisembodiment;

FIG. 11 is a cross-sectional view of a disc rotation apparatus using thehydrodynamic bearing in accordance with this embodiment of the presentinvention;

FIG. 12 is the cross-sectional view showing the right half of theconventional hydrodynamic bearing;

FIG. 13 is the plan view of the flange 33 of the conventionalhydrodynamic bearing;

FIG. 14 is the bottom view of the flange 33 of the conventionalhydrodynamic bearing;

FIG. 15 is the cross-sectional view of a relevant part showing thedistribution of oil pressure generated by the dynamic pressuregeneration grooves 33A and 33B of the flange 33 in the conventionalhydrodynamic bearing; and

FIG. 16 is the cross-sectional view of a relevant part showing thedistribution of oil pressure generated in the radial direction by thedynamic pressure generation grooves 32B and 3C of the sleeve 32 of theconventional hydrodynamic bearing.

DESCRIPTION OF PREFERRED EMBODIMENT

A preferred embodiment of a hydrodynamic bearing in accordance with thepresent invention will be described below referring to FIGS. 1 to 10.FIG. 1 is a cross-sectional view of a hydrodynamic bearing in accordancewith an embodiment of the present invention. In FIG. 1, a sleeve 1 has abearing hole 20 at its nearly central portion, and herringbone-shapeddynamic pressure generation grooves 1A and 1B are formed on the innercircumferential face of the bearing hole 20. A recess portion 1C isformed at the lower end of the sleeve 1. A shaft 2 is rotatably insertedinto the bearing hole 20. A flange 3 is secured to the lower end of theshaft 2 so as to be accommodated in the recess portion 1C at the lowerend of the sleeve 1. A thrust plate 4 is secured to the recess portion1C of the sleeve 1 by a securing method, such as laser welding,precision crimping or bonding, and the recess portion 1C including theflange 3 is hermetically sealed. The sleeve 1 is secured to a base 6.The shaft 2 is secured to a hub rotor 7. Dynamic pressure generationgrooves are provided on one of the opposed faces of the flange 3 and thethrust plate 4. In FIG. 1, dynamic pressure generation grooves 3A areprovided on the lower face of the flange 3. Dynamic pressure generationgrooves 3B are also provided on the upper face of the flange 3 opposedto the recess portion 1C of the sleeve 1. The insides of the dynamicpressure generation grooves 1A, 1B, 3A and 3B are filled with oil orgrease. A rotor magnet 9 is installed in the hub rotor 7. In addition, astator 8 is installed on the base 6 so as to be opposed to theabove-mentioned rotor magnet 9. Two discs 10, for example, are installedon the hub rotor 7 via a spacer 12. The discs 10 are secured by aclamper 11 installed on the shaft 2 by a screw 13.

The operation of the hydrodynamic bearing in accordance with thisembodiment configured as mentioned above will be described by withreference to FIGS. 1 to 10. In FIG. 1, first, when electric power isapplied to the coil of the stator 8, a rotating magnet field isgenerated, and the rotor magnet 9 receives a rotation force, and the hubrotor 7, the shaft 2 and the discs 10 rotate together with the damper 11and the spacer 12. By the rotation, the dynamic pressure generationgrooves 1A, 1B, 3A and 3B rake up oil, and pressures are generatedbetween the dynamic pressure generation grooves 1A and 1B and the shaft2 and between the dynamic pressure generation grooves 3A and the thrustplate 4. Hence, the shaft 2 is floated in the upward direction of thefigure and rotates without making contact with the thrust plate 4 andthe sleeve 1.

FIG. 2 is a view of the lower face of the flange 3, that is, the bottomface thereof opposing to the thrust plate 4, and the black-coloredportions indicate the dynamic pressure generation grooves 3A. Theoutside diameter of the pattern of the dynamic pressure generationgroove 3A is designated as d1 o, the inside diameter thereof isdesignated as d1 i and the diameter of the turn-back part is designatedas d1 m. When the flange 3 rotates inside the recess portion 1C of thesleeve 1, an oil pressure G is generated on the face of the flange 3 inthe direction from the outer circumference to the inner circumferencethereof. Furthermore, an oil pressure H is also generated in thedirection from the inner circumference to the outer circumferencethereof. The diameter of the turn-back part wherein the pressure Gbecomes equal to the pressure H is represented by dsy. Usually, thedynamic pressure generation grooves 3A are designed so that the pressureG becomes equal to the pressure H. For this purpose, the diameter d1 mis determined by equation (1), a well-known equation in hydrodynamics.d1m={(d1i ² +d1o ²)/2}^(1/2)  (1)

However, the hydrodynamic bearing in accordance with the presentinvention is designed so that the pressure G becomes larger than thepressure H. This is represented by equations (2) and (3).d1m=dsy−(dsy−d1i)×Y  (2)dsy={(d1i ² +d1o ²)/2}^(1/2)  (3)

In equation (2), Y is designed so as to be in the range of 0.05 to 1.0.

FIG. 3 is a plan view of the flange 3, and the black-colored portionsindicate the dynamic pressure generation grooves 3B. The dynamicpressure generation grooves 3B are designed so that the pressure in thedirection indicated by arrow E from the inner circumference to the outercircumference is nearly balanced with the pressure in the directionindicated by arrow F from the outer circumference to the innercircumference. In other words, when the outside diameter of the patternof the dynamic pressure generation groove 3B is designated as d2 o, theinside diameter thereof is designated as d2 i and the diameter of theturn-back part thereof is designated as d2 m, a relationship representedby equation (4) is established.d2m={(d2o ² +d2i ²)/2}^(1/2)  (4)

The vertical axis of the graph in FIG. 4 represents an oil pressure(pascal) in the dynamic pressure generation groove 3A, which is variabledepending on the value of the above-mentioned Y. The horizontal axisrepresents the value of equation (dsy−d1 m)/(dsy−d1 i). If asymmetry isinsufficient in the pressures inside the bearing, a partially negativepressure portion is generated somewhere inside the bearing, and air maybe accumulated there. On the other hand, if asymmetry is excessive, theinternal pressure becomes too high, and there arises a danger of causingcavitation or microbubbles. Relating to the hydrodynamic bearing inaccordance with this embodiment, a hydrodynamic bearing is made by usingtransparent materials for the sake of observation, and experiments arecarried out. As a result, it was found that when the value of theabove-mentioned Y was in the range of 0.05 to 1.0, the amount of airbubbles entered and the amount of air accumulated during rotation wereminimal, whereby this range was an appropriate range and air is leastlikely to be accumulated in oil.

FIG. 5 is a cross-sectional view showing the cross-section of a relevantpart and the pressure distribution of oil by the dynamic pressuregeneration grooves 3A and 3B with reference to the atmospheric pressurein the case that the floating amount (S1) of the flange 3 from thethrust plate 4 is sufficiently small. In the hydrodynamic bearing inaccordance with the present invention, only the positive pressureindicated by curve P10 representing the pressure distribution of oil isgenerated and no negative pressure is generated. For this reason, aphenomenon of air accumulation between the flange 3 and the thrust plate4 hardly occurs.

FIG. 6 is a cross-sectional view showing the cross-section of a relevantpart and the pressure distribution of oil by the dynamic pressuregeneration grooves 3A and 3B as indicated by pressure curves P11 and P12in the case that the floating amount (S2) is sufficiently large. Even inthis case, no negative pressure is generated inside the bearing asindicated by the pressure curve P11. In FIG. 6, the positive pressureindicated by the curve P12 of the pressure generated by the dynamicpressure generation grooves 3B on the upper face of the flange 3prevents collision between the flange 3 and the sleeve 1.

FIG. 7 and FIG. 8, views showing the cross-sections of a relevant partand the pressure distributions, show detailed pressure distributionsregarding the pressures generated in the radial direction (theleft-to-right direction in the figure) of the dynamic pressuregeneration grooves 1A and 1B. FIG. 7 shows a case wherein the clearanceportions of the hydrodynamic bearing are wholly filled with oil 5 andthe liquid face is above the upper ends of the dynamic pressuregeneration grooves 1A. The dynamic pressure generation grooves 1A areprovided in the upper portion of the sleeve 1 and made asymmetric suchthat the groove portion 28A in the range of the upper half dimension L1is longer than the groove portion 29A in the range of the lower halfdimension L2. Hence, the oil is pressed downward by the effect ofdynamic pressure, thereby being prevented from leaking outside. Theacute connection part of the groove portion 28A and the groove portion29A is referred to as a turn-back part. The groove portion 28A and thegroove portion 29A of the dynamic pressure generation groove 1A have thesame inclination angle. In the configuration shown in FIG. 7, if thedifference between the dimension L1 and the dimension L2 of the dynamicpressure generation groove 1A is too small, there is a danger of causingoil leakage. On the other hand, if the difference is too large, theinternal pressure becomes too high, and there is a danger of generatingcavitation or microbubbles.

In the dynamic pressure generation groove 1B, the groove portion 28B ofthe upper half is made symmetric with the groove portion 29B of thelower half. Since the dynamic pressure generation groove 1A is madeasymmetric, the pressure inside the bearing becomes positive asindicated by pressure curve P13. Since no negative pressure is generatedinside the bearing even in this case, air accumulation hardly occurs.The pressures in the thrust direction become positive as indicated bypressure curves P14 and P15, whereby no negative pressure is generated.

FIG. 8 shows a case wherein the oil inside the bearing decreases andbecomes insufficient by the amount corresponding to the dimension ΔL.Even in this case, only the positive pressure is generated as indicatedby pressure curve P17, whereby no negative pressure is generated insidethe bearing.

FIG. 9 shows the appropriate range of the asymmetry of the dynamicpressure generation groove 1A. It is desirable that the dimension L2 ofthe groove portion 29A is smaller than the dimension L1 of the grooveportion 28A, that is, the portion on the opposite side thereof, and thatthe value of the relational expression shown on the left side ofequation (5) with respect to the dimensions L1 and L2 is in the range ofvalues shown on the right side.(L1+L2)/(2×L2)=1.02 to 1.60  (5)

In the range indicated in equation 5, the entry of air and the entry ofmicrobubbles hardly occurred.

FIG. 10 shows the relationship between the kinematic viscosity of oil orthe kinematic viscosity of the base oil of grease and the bubble mixingrate into the clearances of the bearing, obtained from the observationresults of the experimental bearing made of the transparent materials.The bubble mixing rate is represented by the percentage of the volume ofbubbles with respect to the volume of oil. According to the observationresults, it was found that the bubble mixing rate was very low in thecase when oil or the base oil of grease had a kinematic viscosity of 4cSt or more at 40° C. of temperature.

The configuration and operation of a disc rotation apparatus using thehydrodynamic bearing in accordance with the present invention will bedescribed by using FIG. 11. In FIG. 11, on a hydrodynamic bearingprovided inside a box-shaped base 6 and comprising a sleeve 1, a shaft2, a flange 3, a thrust plate 4, a hub rotor 7, a stator 8 and a rotormagnet 9, two discs 10 are installed while space is providedtherebetween by using a spacer 12. Heads 25 respectively supported byarms 15 are opposed to both faces of the disc 10. The arms 15 rotatewhile being supported by a head support shaft 16. The upper face of thebase 6 is hermetically sealed by an upper lid 14 so as to prevent theentry of dust and the like. When electric power is applied to the motorstator 8, a rotating magnet field is generated, and the rotor magnet 9starts rotating together with the hub rotor 7, the shaft 2 and the discs10. The dynamic pressure generation grooves 1A, 1B, 3A and 3B rake upoil by pumping forces and generate pressures, whereby the bearingportion floats and rotates with high accuracy in a noncontact state. Theheads 25 make contact with the rotating discs 10, thereby recording orreproducing electrical signals.

Although the thrust plate is secured to the sleeve 1 in FIG. 1, it maybe secured to the base 6 if the interior of the bearing can behermetically sealed.

Even if helical dynamic pressure generation grooves, in which d1 m=d1 o,are used as a modification application example of the dynamic pressuregeneration grooves 3A shown in FIG. 2, instead of the herringbone-shapedgrooves, nearly equivalent performance can be obtained.

As mentioned above, with the hydrodynamic bearing in accordance withthis embodiment, the entry of air into the hydrodynamic bearing sectionis prevented, and the breakage of oil film, having been apt to occur inbearings, is prevented. As a result, a long-life disc rotation apparatuscapable of rotating discs with high accuracy is obtained by using thehydrodynamic bearing in accordance with the present invention.

In addition, the design conditions of the dynamic pressure generationgrooves are combined with the selection conditions of the kinematicviscosity of oil so that the accumulation of air inside the bearing dueto the pumping forces in the dynamic pressure generation grooves isprevented during rotation, therefore the breaking of oil film in theclearances of the bearing does not occur, whereby the hydrodynamicbearing in accordance with the present invention has high accuracy andlong life.

1. A hydrodynamic bearing comprising: a sleeve having a bearing hole atthe nearly central portion thereof, a shaft rotatably inserted into saidbearing hole of said sleeve, and a nearly disc-shaped flange secured toone end of said shaft, one face of said flange opposing to the end faceof said sleeve and the other face thereof opposing to a thrust plateprovided to hermetically seal a region including said end face of saidsleeve, wherein herringbone-shaped first and second dynamic pressuregeneration grooves are provided on at least one of the innercircumferential face of said sleeve and the outer circumferential faceof said shaft so as to be arranged in the direction along said shaft,herringbone-shaped third dynamic pressure generation grooves areprovided on at least one of the opposed faces of said flange and saidthrust plate, said first, second and third dynamic pressure generationgrooves are filled with oil having a kinematic viscosity of 4 cSt ormore at 40° C. of temperature, one of said sleeve and said shaft issecured to a base and the other is secured to a rotatable hub rotor, andwhere an outside diameter of the herringbone pattern of said thirddynamic pressure generation groove is designated as d1 o, an insidediameter thereof is designated as d1 i, a diameter of the turn-back partof the herringbone pattern is designated as d1 m, wherein a diameterwhere the oil pressure generated by said third dynamic pressuregeneration grooves in the direction from the outer circumference to theinner circumference of said flange equals the oil pressure generated inthe direction from the inner circumference to the outer circumferencethereof is designated as dsy, the diameter d1 m of said turn-back partis determined so that the following two equations are satisfiedrespectively:d1m=dsy−(dsy−d1i)×Adsy={(d1i ² +d1o ²)/2}^(1/2) wherein A is a value of 0.05 or more toless than 1.0.
 2. A hydrodynamic bearing comprising: a sleeve having abearing hole at the nearly central portion thereof, a shaft rotatablyinserted into said bearing hole of said sleeve, and a nearly disc-shapedflange, secured to one end of said shaft, one face of said flangeopposing to the end face of said sleeve and the other face thereofopposing to a thrust plate provided to hermetically seal a regionincluding said end face of said sleeve, wherein herringbone-shaped firstand second dynamic pressure generation grooves are provided on at leastone of the inner circumferential face of said sleeve and the outercircumferential face of said shaft, among said first and second dynamicpressure generation grooves, where the grooves away from said thrustplate are designated as said first dynamic pressure generation groovesand the grooves close thereto are designated as said second dynamicpressure generation grooves, a first length L1 of the groove portionwhich is away from said thrust plate in said herringbone-shaped firstdynamic pressure generation groove in the direction of said shaft islarger than a second length L2 of the groove portion which is close tosaid thrust plate in the direction of said shaft, and the value of acalculation expression, (L1 +L2)/(2×L2) represented by said first lengthL1 and said second length L2, is in the range of 1.02 to 1.60, saidherringbone-shaped second dynamic pressure generation groove is madesymmetric with respect to a line passing through herringbone-shapedturn-back parts, herringbone-shaped third dynamic pressure generationgrooves are provided on at least one of the opposed faces of said flangeand said thrust plate, said first, second and third dynamic pressuregeneration grooves are filled with oil having a kinematic viscosity of 4cSt or more at 40° C. of temperature, one of said sleeve and said shaftis secured to a base and the other is secured to a rotatable hub rotor,and where an outside diameter of the herringbone pattern of said thirddynamic pressure generation groove is designated as d1 o, an insidediameter thereof is designated as d1 i, a diameter of the turn-back partof the herringbone pattern is d1 m, wherein the oil pressure generatedby said third dynamic pressure generation grooves in the direction fromthe outer circumference to the inner circumference of said flange equalsthe oil pressure generated in the direction from the inner circumferenceto the outer circumference thereof is designated as dsy, the diameter d1m of said turn-back part is determined so that the following equationsare satisfied respectively:d1m=dsy−(dsy−d1i)×Adsy={(d1i ² +d1o ²)/2}^(1/2) wherein A is a value of 0.05 or more toless than 1.0.
 3. A hydrodynamic bearing in accordance with claim 1,wherein herringbone-shaped fourth grooves are provided on at least oneof the opposed faces of said flange and said sleeve, and where theoutside diameter of the herringbone pattern of said fourth groove isdesignated as d2 o, the inside diameter thereof is designated as d2 iand the diameter of the turn-back part is designated as d2 m, arelationship represented by d2 m={(d2 i ²+d2 o ²)/2}^(1/2) is satisfied.4. A disc rotation apparatus for recording or reproducing signals,wherein a recording/reproduction disc is concentrically secured to saidhub rotor of said hydrodynamic bearing in accordance with claim 1 androtated, magnetic heads or optical heads are provided so as to beopposed to the faces of said rotating disc, and said magnetic heads oroptical heads are configured so as to be movable in parallel with thefaces of said disc.